In a typical horizontal deflection circuit of a television receiver, the trace switch of the horizontal deflection generator applies a trace voltage developed across a trace capacitor to the horizontal deflection winding to generate an S-shaped horizontal deflection or scanning current. If dissipative losses in the horizontal deflection winding and generator were absent, the S-shaped scanning current would be a relatively symmetrical waveform. Due to dissipative losses in such elements as the trace switch and horizontal deflection winding, the scanning current waveform becomes asymmetrical, having a shallower slope than the ideal towards the end of the trace interval. Linearity distortion of the displayed raster is introduced, with the second half of each raster line being compressed relative to the first half.
To provide linearity correction of the scanning current waveform, some conventional horizontal deflection circuits include a variable impedance in series with the trace capacitor and deflection winding. The impedance of the linearity correction element is controlled so that the horizontal deflection current flowing in the impedance element encounters a smaller impedance during the second half of trace than during the first half of trace. Such a controllable impedance may comprise, for example, a saturable reactor winding having a core appropriately biased by a permanent magnet.
For television receiver picture tubes requiring large angle deflection angles, such as 100.degree. or 110.degree., it may be difficult to design a relatively compact and inexpensive saturable reactor which provides the required linearity correction. Furthermore, since the average impedance of the saturable reactor for large angle deflection may comprise a substantial fraction of the impedance of the horizontal deflection winding, the reactor must be designed to withstand a proportionally higher power dissipation as well as the stress imposed by a relatively large retrace pulse voltage developed across the reactor winding.
A circuit described in U.S. patent application Ser. No. 174,864, entitled "Linearity Corrected Deflection Circuit" filed in the name of D. H. Willis and herein incorporated by reference provides linearity correction to horizontal scanning current without using a controllable impedance. A source of horizontal rate alternating polarity voltage comprising a square wave is placed in series with the source of trace voltage and the horizontal deflection winding. When the trace switch is made conductive during the trace interval of each horizontal deflection cycle, the voltage applied across the horizontal deflection winding is the sum of the trace voltage and the horizontal rate alternating polarity voltage. A phasing circuit controls the phase of the horizontal alternating polarity voltage such that linearity correction of the horizontal deflection current is obtained. The phasing is such that the sum of the trace voltage and alternating polarity voltage during the latter portion of the trace interval is greater in magnitude than the same sum during the former portion, thereby steepening the slope of the scanning current waveform during the latter portion of trace from what it otherwise would be.
The previously described circuit provides excellent linearity correction with little added expense and few additional parts. However, if the nonlinearity-causing losses are completely replaced via the linearity correction circuit, the current from the source of trace voltage which would otherwise make up the losses may approach zero during periods of the horizontal trace interval, thereby shifting the DC level of the trace voltage waveform. This may cause variations in raster scan width in response to changes in power supply loading, resulting in raster "breathing".